1. Field of the Invention
The invention relates to ceramic network materials used for restorative material and for various devices such as posts, implant abutments, orthodontic brackets, or blocks.
2. Description of Related Art
Referring to FIG. 1, natural tooth enamel 10 is a hard, vitreous substance that covers the outer portions on a tooth crown 1. Hardness is an important enamel property because enamel 10 provides a protective covering for the softer underlying dentin 12. Enamel 10 also serves as a masticatory surface on which food is crushed, ground, and chewed. Specifically, hardness is a measure of the ability of tooth enamel to withstand deformation by indentation or scraping, or the like, and mature enamel demonstrates a Knoop hardness number (KHN, i.e., the ratio of a given load to an area of indentation expressed in Kg/mm.sup.2) in a range of about 200 to 500 KHN.
Because enamel is semitranslucent, the color of enamel depends in part upon its thickness. For this reason, enamel may assume the various colors of its underlying structures. Thus, when enamel is thicker, and consequently more opaque, it may appear grayish or bluish white reflecting more of its inherent coloration. When enamel is relatively thin, however, it may be yellow-white in appearance, reflecting the underlying generally-yellowish dentin.
As demonstrated by FIG. 1, dentin 12 may constitute the largest single component of tooth structure, extending almost the entire length of the tooth. Dentin 12 is covered by enamel 10 on the crown 1 and by cementum 14 on the root. The internal surface of dentin 12 forms the walls of a pulp cavity 16 which primarily contains pulpal tissue 18. Further, the walls of pulp cavity 16 may closely conform to the outline of the external surface of the dentin 12.
Dentin and bone in general are natural ceramic-composites. Chemically, dentin 12 is composed of organic and inorganic matter. As noted above, the inorganic matter includes calcium phosphate in the form of hydroxyapatite (Ca.sub.10 (PO.sub.4).sub.6 (OH.sub.2)). The organic matter is primarily collagenous material. The hydroxyapatite crystals are bonded to themselves to create tubules through which collagen fibers run and these may be attached to the dentin.
Restorative materials generally are tested on three criteria: sufficient hardness, adequate fit, and acceptable aesthetics, e.g., color match. Of these three, however, aesthetics often are the deciding factor in the choice of restorative materials. Composite resins and ceramics are in widespread use due in part to their ability to match the color of a patient's natural teeth. Composite resins may be composed of a glass in a polymer matrix. This combination, however, may result in rapid wear of the restoration, as the softer polymer is lost, and the glass filler pulls out of the remaining polymer. High wear rates are associated with rapid loss of restorative strength. Restorative ceramics may also be problematic. Despite relatively high strength and hardness, ceramic materials also are generally brittle, such that they may withstand only minimal deformation without failing. Thus, wear in current composite resin materials and catastrophic fracture of ceramic restorations are significant limitations of currently available restorative materials.
On the other hand, direct filling composite resin restorations are widely used to fill decayed teeth. These materials consist of a glass and/or ceramic particles placed into a resin to create a paste. The paste is placed directly into the tooth and cured.
Orthodontics involves movement of teeth by applying force to the teeth via wires which are tied to brackets mounted on the teeth. Most brackets are fabricated from metal which is not aesthetic. New brackets fabricated from alumina are more aesthetic but tend to fracture prematurely and also wear the opposing teeth. Alumina brackets also have high friction with the archwires which slows the tooth movement and prolongs treatment time. Plastic brackets deform and thus decrease the force transmitted to the teeth, prolonging treatment.